The invention pertains to methods and apparatus for optical data storage.
Optical data storage systems use changes in absorption, reflection, and/or refractive index of a storage material to store and retrieve data. In conventional optical data storage systems, individual spatial locations (xe2x80x9ccellsxe2x80x9d) on a substrate are allocated to store individual bits. A sequence of data bits is recorded in such memory systems by mapping each bit onto a different storage cell, and changing a material parameter at each cell to represent the data bit. Readout of the memory is accomplished by illuminating each cell with a light source in conventional implementations of such memories (CD-ROM, magneto-optic disks, etc.), each cell records one data bit.
Rather than directly allocating each cell to an individual data bit, frequency-selective data storage (xe2x80x9cFSDSxe2x80x9d) memories have been demonstrated in which multiple data bits are recorded in each storage cell. Volatile frequency-selective memories are disclosed in, for example, U.S. Pat. No. 3,896,420, and non-volatile memories are disclosed in, for example, U.S. Pat. No. 4,101,976. FSDS memories record multiple bits in each cell using materials that allow spectral addressing of individual atoms molecules. FSDS memories thus use both spectral and spatial addressing to access different portions of the storage material""s absorption spectrum as well as different locations in the storage material.
FSDS systems use storage materials that have inhomogeneously broadened absorption profiles such as the absorption profile 101 of FIG. 1. An absorption profile of an inhomogeneously broadened material (such as the absorption profile 101) is characterized by an inhomogeneous linewidth xcex94xcexdi that is a measure of the spectral width of the absorption profile (typically the full width of the absorption profile at one-half of the maximum value of absorbance). The absorption profile results from a summation of absorption profiles from individual absorbers (atoms, molecules or other active absorber centers), each having a spectral absorption profile and a spectral width referred to as a homogeneous absorption profile and a homogeneous linewidth xcex94xcexdh, respectively. FIG. 2 shows an example of a homogeneous absorption profile 201 of an individual absorber. Inhomogeneous broadening arises from the differing microenvironments for individual absorbers shifting the optical frequencies at which absorption occurs. Thus, the inhomogeneous absorption profile represents a combination of narrower, homogeneous absorption profiles centered at different frequencies. FSDS systems use materials in which the inhomogeneous linewidth is larger than the homogeneous linewidth.
When an inhomogeneously broadened material is illuminated with a single frequency light source, only the absorbers resonant with this single frequency interact with the light, resulting in optical excitation of these absorbers. Illuminating such a material with light having a bandwidth less than the material""s inhomogeneous absorption linewidth produces a dip, or xe2x80x9cspectral holexe2x80x9d in the absorption profile. The minimum width of a spectral hole is approximately equal to the homogeneous absorption linewidth. FIG. 3 illustrates a spectral hole 301 of width xcex94xcexdh in an inhomogeneous absorption profile 303. FSDS systems use multiple spectral holes to record multiple bits in a single cell. The number of spectral storage channels available in a single cell of an inhomogeneously broadened material is determined by the ratio xcex94xcexdi/xcex94xcexdh of the inhomogeneous linewidth xcex94xcexdi to the homogeneous linewidth xcex94xcexdh. The number of spectral channels used is referred to as xe2x80x9cspectral multiplicity.xe2x80x9d For additional discussion of spectral hole-burning, see, for example, W. E. Moerner, ed, Persistent Spectral Hole Buring: Science and Applications (Springer Verlag, New York, 1988).
Two types of FSDS systems have been demonstrated and both can achieve the same spectral multiplicity. The first type is referred to as xe2x80x9cfrequency-domainxe2x80x9d FSDS, and the second class is referred to as xe2x80x9ctime-domain FSDS.xe2x80x9d These two types are discussed briefly below. In addition to these two types of FSDS systems, a xe2x80x9cswept-carrierxe2x80x9d system is disclosed in Mossberg, U.S. Pat. No. 5,276,637, incorporated herein by reference.
Frequency-domain FSDS systems directly address individual spectral channels in an inhomogeneously broadened material. In such systems, a narrowband light source having a spectral width less than the inhomogeneous linewidth xcex94xcexdi illuminates a storage material. A continuous wave (xe2x80x9cCWxe2x80x9d) laser is typically used as the narrow-band source. Absorbers which the narrow-band light source fulfills the resonant condition are excited, recording data. Photo-induced absorption or refractive index changes produced by this excitation are probed to retrieve recorded data. If the linewidth xcex94xcexdl of the narrow-band light source is less than the homogeneous linewidth xcex94xcexdh, the achievable storage capacity in each cell is xcex94xcexdi/xcex94xcexdh. If the source linewidth xcex94xcexdl is larger than the homogeneous linewidth xcex94xcexdh, then the storage capacity is instead xcex94xcexdi/xcex94xcexdl and is said to be xe2x80x9claser linewidth limited.xe2x80x9d
Frequency-domain FSDS imposes data-rate limitations on single bit recording. A spectral channel width xcex94xcexdch must be addressed with illumination having a pulse duration greater than 1/xcex94xcexdch because of a Fourier-transform relationship between pulse duration and linewidth. Thus, to access the kHz-scale linewidths available in some rare-earth-doped crystals, recording and readout pulses of approximately millisecond durations are required. The spectral holes produced in such FSDS systems can be either transient or permanent, as disclosed in U.S. Pat. No. 3,896,420, incorporated herein by reference.
Rather than allocating individual frequency channels to individual bits, time-domain FSDS systems use pulses with spectral widths larger than the homogeneous linewidth xcex94xcexdh and therefore can use pulses with durations less than 1/xcex94xcexdh. Time-domain FSDS systems can record data streams containing pulses that are as short as 1/xcex94xcexdi. In time-domain FSDS systems, a storage material is exposed to a brief reference pulse and a data pulse corresponding to a data-bit stream. These pulses illuminate the storage material sequentially to record an interference between the frequency spectra of the reference pulse and the data pulse, resulting in the direct recording of the spectrum of the data-bit stream. If the reference pulse precedes the data pulse, subsequent illumination of the storage material with a replica of the reference pulse produces a reconstruction of the data pulse. Such time-domain FSDS systems are described in, for example, U.S. Pat. No. 4,459,682, incorporated herein by reference.
Time-domain FSDS systems use temporally distinct reference pulses to record the spectrum of a data-bit stream, while swept-carrier FSDS systems record the spectrum of a data-bit stream using frequency-swept (chirped) reference and data beams. The reference and data beams simultaneously illuminate the storage material, and subsequent illumination with the frequency swept reference beam reproduces the data beam. Such systems are disclosed in, for example, Mossberg, U.S. Pat. No. 5,276,637 and Mossberg et al., Opt. Lett. 17, 535 (1992).
In conventional FSDS systems, a positioning system directs a laser beam to a particular cell, and data is recorded in, or read from, the entire spectral capacity at the cell. Thus, the laser is stationary in two spatial dimensions while the data is stored or retrieved using a third dimension (frequency).
An important limitation of both time-domain and swept-carrier data storage is excitation-induced frequency shifts, also referred to as excitation-induced dephasing or instantaneous dephasing, as described in, for example, Huang et al., Phys. Rev. Lett. 63, 78 (1989). The excitation-induced frequency shifts increase the homogeneous linewidth xcex94xcexdh with increasing levels of illumination. Thus, as data is recorded, the data storage capacity of the material decreases. The storage capacity can be dramatically lower than the intrinsic storage capacity. For example, in Eu3+xe2x80x94doped Y2SiO5, the intrinsic data storage capacity based on the ratio of the inhomogeneous and homogeneous linewidths is greater than 106 bits/cell. See, for example, R. Equall et al, Phys. Rev. Lett. 72, 2179 (1994), Yano et al, J. Opt. Soc. Am. B 9, 992 (1992). The storage capacity of Eu3+xe2x80x94Y2SiO5: drops to approximately 2000 bits/cell whenever the inhomogeneous absorption profile of the storage material is fully excited. Thus, while conventional time-domain data-access methods provide fast data access, excitation-induced frequency shifts severely limit data-storage capacity.
Methods of storing data in cells of a storage material having an inhomogeneous absorption with an inhomogeneous linewidth xcex94xcexdi, are provided. The methods include directing a reference pulse of electromagnetic radiation and a data pulse of electromagnetic radiation to the storage material. The data pulse is modulated according to data to be stored in the cells of the storage material. The reference pulse and the data pulse are spatially-spectrally swept, causing data to be stored in the cells of the storage material. In some embodiments, the reference spatial-spectral trajectory and the data spatial-spectral trajectory are the same while in other embodiments, the reference and data spatial-spectral trajectories are offset in position or frequency. In a representative embodiment, the reference pulse and the data pulse simultaneously illuminate each individual storage cell of the storage material and co-propagate to and through the storage material.
In a specific embodiment, a frequency sweep of at least one of the reference and data spatial-spectral trajectories is a linear sweep.
In other methods, a data bit is stored using a channel bandwidth xcex94xcexdch and at least one of the spatial-spectral trajectories spans a frequency range that is larger than the channel bandwidth xcex94xcexdch.
Methods of storing a first data record and a second data record, the data records including one or more data bits, include providing a storage material having an inhomogeneously broadened absorption of linewidth (xcex94xcexdi) that is greater than the channel bandwidth (xcex94xcexdch) used to store a single data bit. A first reference pulse is provided that has a first starting frequency within the inhomogeneous linewidth (xcex94xcexdi), a first starting spatial position on the storage material, and follows a first spatial-spectral trajectory that spans a first spectral width greater than the channel bandwidth xcex94xcexdch. The channel bandwidth is less than or equal to the inhomogeneous linewidth xcex94xcexdi. A first data pulse is also provided to the storage material. The first data pulse has a second starting frequency within the inhomogeneous linewidth (xcex94xcexdi) and a second starting spatial position on the storage material. The first data pulse follows a second spatial-spectral trajectory that spans a second spectral width that is greater than the channel bandwidth (xcex94xcexdch) and that is less than or equal to the inhomogeneous linewidth (xcex94xcexdi). The first data pulse has a modulation corresponding to a first data record. The first data record is stored in the storage material by exposing the storage material to the first data pulse and the first reference pulse. A second reference pulse and a second data pulse are provided, the second reference pulse and the second data pulse following a third spatial-spectral trajectory and a fourth spatial-spectral trajectory. These trajectories span spectral widths that are greater than the channel bandwidth (xcex94xcexdch) and less than or equal to the inhomogeneous linewidth (xcex94xcexdi). Preferentially, the third spatial-spectral trajectory does not overlap the first spatial-spectral trajectory but does overlap the fourth spectral-spatial trajectory. The second data pulse has a modulation corresponding to a second data record and the second data record is stored in the storage material by exposing the storage material to the second reference pulse and the second data pulse.
In alternative embodiments, the storage material comprises multiple cells for storing portions of the first and second data sequences and the multiple cells are exposed to the first reference pulse and the first data pulse simultaneously. In additional specific embodiments the storage material is Eu3+:YSiO5.
Apparatus for storing and retrieving data from a storage material having an inhomogeneous absorption are provided. The apparatus comprise a laser that produces a laser beam having a frequency that sweeps through a frequency range, a signal generator that generates a reference signal, and a data source that provides a data signal. A modulator such as an acousto-optic modulator, electro-optic modulator or other type of modulator receives the reference signal, the data signal, and the laser beam and generates a reference beam and a data beam. The data beam is modulated by the data signal, and the data beam and the reference beam are co-propagating. A scanner scans the data beam and the reference beam across the storage material, so as to cause data to be stored in cells of the storage material. A detector is provided that receives the reference beam transmitted by a cell and a reconstructed data beam produced by the transmission of the reference beam through the cell. The detector producing a heterodyne signal from the reference and reconstructed data beams, wherein the heterodyne signal has a modulation corresponding to data retrieved from the storage material.